The invention is related to the optimization of power demand control in the systems employing alternating current electrical motors with variable frequency drives (VFDs) serving HVAC and mechanical system pumps, fans, chillers, cooling towers, boilers, air compressors and various manufacturing machines, equipment and processes. During many years VFDs have been widely used to control multiple processes served by electrical motors. VFDs extensive applications have increased ever so more recently due to the strong emphasis on energy conservation. When the electrical motors are running via variable frequency drive control mode at the reduced as compared to the design conditions the motors voltage, operational speeds, power demand and electrical energy consumption are also reduced. However, when a VFD is running the motor at or near its design load then the system power demand and electrical energy consumption is increased as compared to the constant frequency and speed mode due to the inefficiency imposed by the VFD operation. The invention develops the system and control strategy method to switch the VFDs to the bypass operation with constant motor speed and frequency at the constant power line voltage when the motor has to be running at or near 100% of the VFD speed and to switch it back to the VFD control mode when the speed of the motor has to be below 100%. The motor load control when the motor is operating at the constant frequency and speed drive mode is accomplished via variable line amperes draw utilized by the motors to satisfy the load. This reduces motor power demand at the reduced loads. VFD efficiency is typically varies from 94% to 97% for the range of the motor's operational speeds of 30% to 100%. The combined efficiency of the electrical motor operation that is equipped with a VFD control assuming all other factors to be equal could be determined as a result of multiplication of the electrical motor and VFD efficiencies.
The inefficiency imposed by the VFD operations during the most time is compensated with the electrical energy savings due to the reduced speed of the electrical motor. However, when the electrical motor speed is approaching its design magnitude the power demand and energy usage for the equipment with VFD control exceeds their magnitudes for the electrical motors without VFD control when the electrical motor is operating at approximately constant speed. Usually, the electrical motor without VFD control that is running at its design 100% speed will require power demand by about 2.5% to 3% lower than one with VFD control that is also running at 100% speed.
These potential savings are based on the advertised by VFD manufacturers efficiencies for the variable frequency drives at 100%. The actual power demand reduction for the motor at its design speed operating conditions with constant frequency drive mode without VFD control due to a number of factors (i.e., the electrical motors operating at a higher than design loads conditions in the zone of their service factor, deteriorating over time operational efficiency of the VFD drives, etc.) may exceed the shown above magnitudes.
In some current applications VFDs employ a bypass to switch from variable to constant speed mode in case of the VFD failure. However, this provision is done only to improve reliability of motor operation and does not allow to reduce motor power demand or to save energy.